Eddy current flaw detection probe and eddy current flaw inspection apparatus

ABSTRACT

The eddy current flaw detection probe ( 100 ) includes a first yoke ( 1 ), a second yoke ( 2 ), a first permanent magnet ( 3 ), a second permanent magnet ( 4 ), a sensing coil ( 6 ). The second yoke ( 2 ) is inserted in a freely insertable and extractable manner along an inner peripheral surface of the first yoke ( 1 ).

TECHNICAL FIELD

The present invention relates to an eddy current flaw detection probeand an eddy current flaw inspection apparatus having the said eddycurrent flaw detection probe.

BACKGROUND ART

An eddy current flaw detection is known as one of inspection methods forinspecting flaws in metallic materials. In flaw inspection fornon-magnetic pipes composed of non-magnetic materials such as austeniticstainless steels, titanium, and copper alloys, eddy current flawdetection using an eddy current flaw detection probe of insertion typeis widely adopted.

When flaw inspection for a magnetic pipe composed of a magnetic materialsuch as a carbon steel, a ferritic stainless steel, and a duplexstainless steel containing two phases of a ferrite phase and anaustenite phase is conducted by means of the eddy current flaw detectionprobe, used widely for inspection of non-magnetic pipes, accurate flawdetection is not achieved because an eddy current flows merely on thesurface of the magnetic pipe and because a noise caused by localfluctuation in the magnetic permeability affects adversely the flawdetection performance.

Non-patent Document 1 discloses an eddy current flaw detection probeused for a flaw inspection of a magnetic pipe composed of a magneticmaterial of duplex stainless steel. In the eddy current flaw detectionprobe disclosed in Non-patent Document 1, a sensing coil is arrangedaround a center part of a pillar-shaped yoke and then permanent magnetswhose magnetization directions are in the radial direction of the yokeand whose magnetic poles on the yoke side are different from each otherare arranged around the yoke on both sides of the sensing coil.

Non-destructive Inspection, Volume 42, No. 9, 1993 (Heisei 5), pp. 520to 526

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In such an eddy current flaw detection probe, for the purpose ofachieving accurate flaw inspection, the sizes of the permanent magnets,the yokes, and the like, the arrangement positions of the permanentmagnets, and the like are designed with taking into considerationpermeabilities, thicknesses, and the like of magnetic pipes as aninspection target.

In magnetic pipes, variations in magnetic permeability and thickness aregenerally caused by differences in the manufacturing process and thelike. Then, even in a case of magnetic pipes having the same innerdiameter as each other, when the specification of magnetic permeabilityor thickness are different from each other, the optimal specificationfor the eddy current flaw detection probe becomes different from eachother. Thus, when the above-mentioned variation occurs in the magneticpipes, the design of the eddy current flaw detection probe concerningsuch as the sizes of the permanent magnets, the yokes and the like, orthe arrangement positions of the permanent magnets and the like, shouldbe changed for each case.

An object of the present invention is to provide: an eddy current flawdetection probe provided with a mechanism that the magnetic flux densityof a magnetic pipe as an inspection target can be varied by means ofadjusting magnetizing force of the current flaw detection probe withoutrequiring design changes in the sizes of the permanent magnets, theyokes and the like, or in the arrangement positions of the permanentmagnets; and an eddy current flaw inspection apparatus having this eddycurrent flaw detection probe.

Means for Solving the Problem

The present invention is an eddy current flaw detection probecomprising:

a first yoke having a hollow-cylindrical shape and composed of amagnetic material;

a second yoke having a hollow-cylindrical or pillar shape, composed of amagnetic material, and inserted in a freely insertable and extractablemanner along an inner peripheral surface of the first yoke;

a first permanent magnet mounted along an outer peripheral surface ofone end part of the axis direction of the first yoke and polarized in aradial direction of the first yoke;

a second permanent magnet mounted along an outer peripheral surface ofthe other end part of the axis direction of the first yoke, polarized ina radial direction of the first yoke, and magnetized such that themagnetic poles are different from those of the first permanent magnet;and

a sensing coil wound along the outer peripheral surface in a center partof the axis direction of the first yoke and detecting a change in amagnetic flux generated by the first permanent magnet and the secondpermanent magnet.

Further, the eddy current flaw detection probe according to the presentinvention further comprises a magnetic annular body having an annularshape, composed of a magnetic material, and mounted in a freelyattachable and detachable manner along the outer peripheral surface ofthe first yoke.

Further, the present invention is an eddy current flaw inspectionapparatus for inspecting a flaw in a pipe composed of a magneticmaterial, comprising:

the above-mentioned eddy current flaw detection probe moved along theinner peripheral surface of the pipe;

an AC power supply for applying an alternating voltage onto the sensingcoil of the eddy current flaw detection probe;

signal output means for outputting as an electric signal a voltagechange induced on the sensing coil when the alternating voltage isapplied by the AC power supply; and

flaw judging means for judging the presence or absence of a flaw in thepipe on the basis of the electric signal outputted from the signaloutput means.

Effect of the Invention

According to the present invention, the eddy current flaw detectionprobe includes the second yoke having a hollow-cylindrical or pillarshape and inserted in a freely insertable and extractable manner alongthe inner peripheral surface of the first yoke formed in ahollow-cylindrical shape. Thus, when the second yoke is arranged in aninserted manner along the inner peripheral surface of the first yoke,the magnetic flux density is adjusted to a higher value of a magneticpipe as an inspection target in comparison with a case of no second yokeinserted. As such, without requiring a design change concerning thesizes of the first and the second permanent magnets, the first yoke, andthe like, or the arrangement positions of the first and the secondpermanent magnets, the magnetizing force can be adjusted by inserting orextracting the second yoke into or from the first yoke, resulting inchanging the magnetic flux density in the magnetic pipe.

Further, according to the present invention, it is preferable that theeddy current flaw detection probe may further include a magnetic annularbody having an annular shape and mounted in a freely attachable anddetachable manner along the outer peripheral surface of the first yoke.When the magnetic annular body is mounted on the first yoke, themagnetic flux generated by the first and the second permanent magnetsflows through the magnetic annular body, resulting in increasing amagnetic flux leakage. That is, when the magnetic annular body ismounted along the outer peripheral surface of the first yoke, themagnetic flux density of a magnetic pipe as an inspection target isadjusted to a lower value in comparison with a case of no magneticannular body mounted. As such, without requiring a design changeconcerning the sizes of the first and the second permanent magnets, thefirst yoke, and the like, or the arrangement positions of the first andthe second permanent magnets, the magnetizing force can be adjusted byattaching or detaching the magnetic annular body to or from the firstyoke, resulting in changing the magnetic flux density of the magneticpipe.

Further, according to the present invention, since the eddy current flawinspection apparatus has the eddy current flaw detection probe accordingto the present invention, even for magnetic pipes of mutually differentspecifications in the magnetic permeability or the thickness, flawinspection is achieved by using the eddy current flaw detection probewith optimized specification for the specification of the magnetic pipe.Thus, the eddy current flaw inspection apparatus according to thepresent invention provide accurate flaw inspection of a magnetic pipe asan inspection target.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configuration of an eddy current flawdetection probe according to an embodiment of the present invention.

FIG. 2 is a diagram showing a partially sectional exploded side view ofthe configuration of an eddy current flaw detection probe.

FIG. 3 is a graph showing variation in the magnetic flux density and thespecific magnetic permeability of magnetic pipes.

FIG. 4 is a graph showing the relation between the inner diameter of asecond yoke and the magnetic flux density of the magnetic pipe.

FIG. 5 is a graph showing the relation between the length in the axisdirection of a magnetic annular body and the magnetic flux density ofthe magnetic pipe.

FIG. 6 is a diagram showing a configuration of an eddy current flawinspection apparatus according to an embodiment of the presentinvention.

DESCRIPTION OF REFERENCES

1 First yoke

2 Second yoke

3 First permanent magnet

4 Second permanent magnet

5 Third permanent magnet

6 Sensing coil

7 Excitation coil

8 Magnetic annular body

100 Eddy current flaw detection probe

200 Eddy current flaw inspection apparatus

300 Magnetic pipe

DESCRIPTION OF THE EMBODIMENT

FIG. 1 is a diagram showing the configuration of an eddy current flawdetection probe 100 according to an embodiment of the present invention.In FIG. 1, (a) shows a side view and (b) shows a partially sectionalside view. FIG. 2 is a diagram showing a partially sectional explodedside view of the configuration of the eddy current flaw detection probe100.

The eddy current flaw detection probe 100 according to the presentembodiment is a probe to be used for inspecting flaws in the magneticpipe in a manner of moving along the inner peripheral surface of a pipecomposed of a magnetic material (referred to as a “magnetic pipe”,hereinafter). Examples of magnetic pipes as inspection targets includepipes composed of magnetic materials such as carbon steels, ferriticstainless steels, duplex stainless steels containing two phases of aferrite phase and an austenite phase.

The eddy current flaw detection probe 100 includes a first yoke 1, asecond yoke 2, a first permanent magnet 3, a second permanent magnet 4,a third permanent magnet 5, a sensing coil 6, an excitation coil 7,magnetic annular bodies 8, spacers 9, a first lid 10, and a second lid11.

The first yoke 1 is a hollow-cylindrical yoke composed of a magneticmaterial. Examples of magnetic materials for the first yoke 1 includehigh-magnetic permeability metals such as a carbon steel and a low-alloysteel. While the size of the first yoke 1 is not limited, for example,in the case of specification of a carbon steel pipe for boilers and heatexchangers (referred to as an “STB pipe”, hereinafter) widely employedin chemical plants, the length in the axis direction is 50 to 200 mm,the outer diameter is 1.7 to 38.4 mm, and the thickness is 0.5 to 5.0mm.

The second yoke 2 has a hollow-cylindrical or pillar shape and iscomposed of a magnetic material and inserted in a freely insertable andextractable manner along the inner peripheral surface of the first yoke1. The second yoke 2 is used for adjusting the magnetic flux density ofthe magnetic pipe as an inspection target. When the second yoke 2 isinserted along the inner peripheral surface of the first yoke 1, theaxis of the first yoke 1 and the axis of the second yoke 2 align witheach other.

Examples of magnetic materials for the second yoke 2 includehigh-magnetic permeability metals such as a carbon steel and a low-alloysteel. While the size of the second yoke 2 is not limited, for example,in the case of specification of an STB pipe widely employed in chemicalplants, the length in the axis direction is 50 to 200 mm, the outerdiameter is 1.2 to 37.9 mm, and the thickness or the pillar diameter is0.5 to 37.9 mm.

The first permanent magnet 3 is mounted along the outer peripheralsurface of one end part of the axis direction of the first yoke 1. Inthe present embodiment, the first permanent magnet 3 is formed in asingle hollow cylinder shape or in a hollow-cylindrical shapeconstructed by combining a plurality of permanent magnets divided in thecircumferential direction, and is mounted on the first yoke 1 in suchorientation that the axis of the first permanent magnet 3 aligns withthe axis of the first yoke 1. The first permanent magnet 3 is polarizedin the radial direction of the first yoke 1 and magnetized in suchorientation that the magnetic pole on the side of the first yoke 1 is ofan S-pole and the magnetic pole on the opposite side is of an N-pole inthe present embodiment. For example, the first permanent magnet 3 iscomposed of a high-performance permanent magnet such as a neodymiummagnet.

While the size of the first permanent magnet 3 formed in the shape of asingle hollow cylinder or alternatively in a hollow-cylindrical shapeconstructed by combining a plurality of permanent magnets divided in thecircumferential direction is not limited, for example, in the case ofspecification of an STB pipe widely employed in chemical plants, thelength L1 in the axis direction is 5.0 to 50.0 mm, the inner diameter is1.7 to 38.4 mm, and the thickness is 0.5 to 5.0 mm.

The second permanent magnet 4 is mounted along the outer peripheralsurface of the other end part of the axis direction of the first yoke 1.In the present embodiment, the second permanent magnet 4 is formed in asingle hollow cylinder shape or in a hollow-cylindrical shapeconstructed by combining a plurality of permanent magnets divided in thecircumferential direction and is mounted on the first yoke 1 in suchorientation that the axis of the second permanent magnet 4 aligns withthe axis of the first yoke 1. The second permanent magnet 4 is polarizedin the radial direction of the first yoke 1 and magnetized such that themagnetic poles are different from those of the first permanent magnet 3.In the present embodiment, the second permanent magnet 4 is magnetizedin such orientation that the magnetic pole on the side of first yoke 1is of an N-pole and the magnetic pole on the opposite side is of anS-pole. For example, the second permanent magnet 4 is composed of ahigh-performance permanent magnet such as a neodymium magnet.

While the size of the second permanent magnet 4 formed in a singlehollow cylinder shape or in a hollow-cylindrical shape constructed bycombining a plurality of permanent magnets divided in thecircumferential direction is not limited, the preferable size of thesecond permanent magnet 4 is the same as the size of the first permanentmagnet 3. For example, in the case of specification of an STB pipewidely employed in chemical plants, the length L2 in the axis directionis 5.0 to 50.0 mm, the inner diameter is 1.7 to 38.4 mm, and thethickness is 0.5 to 5.0 mm.

The first permanent magnet 3 and the second permanent magnet 4 aremounted along the outer peripheral surface of the first yoke 1 such asto be mutually in the same distance from the center of the first yoke 1in the axis direction of the first yoke 1.

The third permanent magnet 5 may be mounted along the outer peripheralsurface of the center part of the axis direction of the first yoke 1. Inthe present embodiment, for example, the third permanent magnet 5 isformed in a hollow-cylindrical shape and mounted on the first yoke 1 insuch orientation that the axis of the third permanent magnet 5 alignswith the axis of the first yoke 1. The third permanent magnet 5 ispolarized in the axis direction of the first yoke 1 and magnetized insuch orientation that the magnetic pole on the side of the firstpermanent magnet is of an N-pole and the magnetic pole on the secondpermanent magnet 4 side is of an S-pole in the present embodiment. Forexample, the third permanent magnet 5 is composed of a high-performancepermanent magnet such as a neodymium magnet.

When the third permanent magnet 5 is mounted in the center part of theaxis direction of the first yoke 1, the magnetic flux density of themagnetic field generated by the first permanent magnet 3 and the secondpermanent magnet 4 may be increased. When the third permanent magnet 5is magnetized in such orientation that the magnetic pole on the side ofthe first permanent magnet 3 is of an N-pole and the magnetic pole onthe second permanent magnet 4 side is of an S-pole, the magnetic fluxdensity may be increased and the magnetic flux distribution may becomeuniform.

While the size of the third permanent magnet 5 formed in ahollow-cylindrical shape is not limited, for example, in the case ofspecification of an STB pipe widely employed in chemical plants, thelength in the axis direction is 5.0 to 25.0 mm, the inner diameter is1.7 to 38.4 mm and the thickness is 0.5 to 5.0 mm.

The sensing coil 6 is wound along the outer peripheral surface in thecenter part of the axis direction of the first yoke 1. In the presentembodiment, when the third permanent magnet 5 is mounted in the centerpart of the axis direction of the first yoke 1, the sensing coil 6 maybe wound along the outer peripheral surface of the third permanentmagnet 5. The sensing coil 6 detects a change in the magnetic fluxgenerated by the first permanent magnet 3, the second permanent magnet4, and optionally the third permanent magnet 5.

While the sensing coil 6 is not limited to particular one, for example,in the case of specification of an STB pipe widely employed in chemicalplants, a coil which is employed is made from a copper wire of diameterof 0.02 to 1.0 mm and its number of turns is 10 to 200.

The excitation coil 7 may be wound along the outer peripheral surface ofthe third permanent magnet 5, in a manner of pinching the sensing coil6. The function of the excitation coil 7 is to excite an eddy currentflowing in the opposite direction to the eddy current excited by thesensing coil 6. When the excitation coil 7 arranged on the outer side ofthe sensing coil 6 may excite an eddy current in the opposite direction,these eddy currents may cancel with each other, resulting in suppressingan excessive electric conduction area of the eddy current excited by thesensing coil 6.

While the excitation coil 7 is not limited to particular one, forexample, in the case of specification of an STB pipe widely employed inchemical plants, a preferable coil is made from a copper wire ofdiameter of 0.02 to 1.0 mm and its number of turns is 10 to 200.

The magnetic annular bodies 8 are composed of a magnetic material andformed in an annular shape. The magnetic annular bodies 8 are optionallymounted in a freely attachable and detachable manner along the outerperipheral surface of the first yoke 1. The magnetic annular bodies 8may be used for adjusting the magnetic flux density to a lower value inthe magnetic pipe as an inspection target.

Examples of magnetic materials for the magnetic annular bodies 8 includehigh-magnetic permeability metals such as a carbon steel and a low-alloysteel.

While the size of the magnetic annular bodies 8 is not limited, forexample, in the case of specification of an STB pipe widely employed inchemical plants, the length L3 in the axis direction is 0.5 to 50 mm,the inner diameter is 1.7 to 38.4 mm, and the thickness is 0.5 to 5.0mm.

The eddy current flaw detection probe 100 according to the presentembodiment is, for example, provided with two magnetic annular bodies 8.The two magnetic annular bodies 8 may be mounted along the outerperipheral surface of the first yoke 1 such as to be mutually in thesame distance from the center of the first yoke 1 in the axis directionof the first yoke 1. The two magnetic annular bodies 8 may be providedon the outer sides of the first permanent magnet 3 and the secondpermanent magnet 4 in the axis direction of the first yoke 1.

The spacers 9 may be made from any material selected from non-metallicmaterials having a satisfactory sliding property. For example, thespacers 9 are composed of DURACON or a PEEK resin and formed in anannular shape. The spacers 9 may be mounted in a freely attachable anddetachable manner along the outer peripheral surface of the first yoke 1in the outermost parts at both ends of the axis direction of the firstyoke 1.

The first lid 10 is a member optionally mounted in a freely attachableand detachable manner in the opening part in one end part of the axisdirection of the first yoke 1. The first lid 10 is formed, for example,in a closed-end tube shape and has an apex part and a side part. Whenthe first lid 10 is mounted, the first lid 10 is arranged such that theapex part abuts against the end face of the opening part on the one endpart side of the axis direction of the first yoke 1 and the innerperipheral surface of the side part abuts against the outer peripheralsurface of the opening part on the one end part side of the axisdirection of the first yoke 1 so that the opening part of the first yoke1 is closed.

The second lid 11 is a member optionally mounted in a freely attachableand detachable manner in the opening part in the other end part of theaxis direction of the first yoke 1. The second lid 11 is formed, forexample, in a closed-end tube shape and has an apex part and a sidepart. When the second lid 11 is mounted, the second lid 11 is arrangedsuch that the apex part abuts against the end face of the opening parton the other end part side of the axis direction of the first yoke 1 andthe inner peripheral surface of the side part abuts against the outerperipheral surface of the opening part on the other end part side of theaxis direction of the first yoke 1 so that the opening part of the firstyoke 1 is closed.

The eddy current flaw detection probe 100 having the above-mentionedconfiguration includes the second yoke 2 having a hollow-cylindrical orpillar shape and inserted in a freely insertable and extractable manneralong the inner peripheral surface of the first yoke 1 formed in ahollow-cylindrical shape. Thus, when the second yoke 2 is arranged in aninserted manner along the inner peripheral surface of the first yoke 1,the magnetic flux density is adjusted to a higher value in the magneticpipe as an inspection target in comparison with a case of no second yoke2 inserted. Accordingly, without requiring a design change concerningthe sizes of the first permanent magnet 3, the second permanent magnet4, the first yoke 1, and the like, or the arrangement positions of thefirst permanent magnet 3 and the second permanent magnet 4, themagnetizing force can be adjusted by inserting or extracting the secondyoke 2 into or from the first yoke 1, resulting in changing the magneticflux density of the magnetic pipe.

Further, the eddy current flaw detection probe 100 according to thepresent embodiment may further include magnetic annular bodies 8 havingan annular shape and mounted in a freely attachable and detachablemanner along the outer peripheral surface of the first yoke 1. When themagnetic annular bodies 8 is mounted on the first yoke 1, the magneticflux generated by the first permanent magnet 3 and the second permanentmagnet 4 flows through the magnetic annular bodies 8, resulting inincreasing magnetic flux leakage. That is, when the magnetic annularbodies 8 is mounted along the outer peripheral surface of the first yoke1, the magnetic flux density is adjusted to a lower value in themagnetic pipe as an inspection target in comparison with a case of nomagnetic annular body 8 mounted. Accordingly, without requiring a designchange concerning the sizes of the first permanent magnet 3, the secondpermanent magnet 4, the first yoke 1, and the like or the arrangementpositions of the first permanent magnet 3 and the second permanentmagnet 4, the magnetizing force can be adjusted by attachment ordetachment of the magnetic annular bodies 8 to or from the first yoke 1,resulting in changing the magnetic flux density of the magnetic pipe.

In the present embodiment, it is preferable that the magnetic fluxdensity of the magnetic field generated by the first permanent magnet 3and the second permanent magnet 4 is 90% or higher of the maximummagnetic flux density of the magnetic pipe in the center part of thesensing coil 6 with respect to the axis direction of the magnetic pipeas an inspection target. For example, in the case of an STB pipe, it ispreferable that the magnetic flux density is 1.8 T (tesla) or higher.When the magnetic flux density is lower than 1.8 T, the flaw detectionperformance may be degraded in flaw inspection of an STB pipe and thissituation is not preferable.

In the present invention, the values of magnetic flux density can beobtained by numerical analysis of finite element method by usingmagnetic field analysis software “Electromagnetic Field AnalysisSoftware JMAG (registered trademark)” (made by JSOL Corporation).

FIG. 3 is a graph showing variation in the magnetic flux density and thespecific magnetic permeability of magnetic pipes. In FIG. 3, the curveX1 indicates a change in the magnetic flux density caused by thicknessvariation in the magnetic pipes as inspection targets. The plot X2indicates a change in the specific magnetic permeability caused by,thickness variation in the magnetic pipes as inspection targets. Asshown in FIG. 3, the magnetic pipes have variation in the magnetic fluxdensity and in the magnetic permeability which is caused by a differencein the manufacturing process or the like.

Since the eddy current flaw detection probe 100 according to the presentembodiment has the second yoke 2 and optionally the magnetic annularbodies 8 as a mechanism for changing the magnetic flux density in themagnetic pipe as an inspection target, even for magnetic pipes havingmutually different specifications concerning the magnetic permeabilityor the thickness, the specification of the eddy current flaw detectionprobe 100 is easily designed to optimize for the specification of themagnetic pipe.

FIG. 4 is a graph showing the relation between the inner diameter of thesecond yoke 2 and the magnetic flux density of the magnetic pipe. InFIG. 4, the curve Y1 indicates a change in the magnetic flux density ofthe magnetic pipe caused when the inner diameter of the second yoke 2 ischanged.

Under the condition that the lengths in the axis direction of the firstpermanent magnet 3 and the second permanent magnet 4 are set to be 13mm, when the second yoke 2 having an inner diameter of 15 mm is insertedalong the inner peripheral surface of the first yoke 1, the magneticflux density of the magnetic pipe is reduced by 10% in comparison with acase of no second yoke 2 inserted. Under the condition that the lengthsin the axis direction of the first permanent magnet 3 and the secondpermanent magnet 4 are set to be 13 mm, when the second yoke 2 having aninner diameter of 20 mm is inserted along the inner peripheral surfaceof the first yoke 1, the magnetic flux density of the magnetic pipe isreduced by 40% in comparison with a case of no second yoke 2 inserted.Accordingly, the magnetizing force can be adjusted by inserting orextracting the second yoke 2 into or from the first yoke 1, resulting inchanging the magnetic flux density of the magnetic pipe.

FIG. 5 is a graph showing the relation between the length in the axisdirection of magnetic annular bodies 8 and the magnetic flux density ofthe magnetic pipe. In FIG. 5, the curve Y2 indicates a change in themagnetic flux density of the magnetic pipe caused when the length in theaxis direction of the magnetic annular bodies 8 is changed.

Under the condition that the lengths in the axis direction of the firstpermanent magnet 3 and the second permanent magnet 4 are set to be 10mm, when the pair of magnetic annular bodies 8 having an axisdirectional length of 5 mm are mounted along the outer peripheralsurface of the first yoke 1, the magnetic flux density of the magneticpipe is reduced by 5% in comparison with a case of no magnetic annularbody 8 mounted. Further, under the condition that the lengths in theaxis direction of the first permanent magnet 3 and the second permanentmagnet 4 are set to be 10 mm, when the pair of magnetic annular bodies 8having an axis directional length of 10 mm are mounted along the outerperipheral surface of the first yoke 1, the magnetic flux density of themagnetic pipe is reduced by 6.5% in comparison with a case of nomagnetic annular body 8 mounted. Further, under the condition that thelengths in the axis direction of the first permanent magnet 3 and thesecond permanent magnet 4 are set to be 10 mm, when the pair of magneticannular bodies 8 having an axis directional length of 20 mm are mountedalong the outer peripheral surface of the first yoke 1, the magneticflux density of the magnetic pipe is reduced by 7.8% in comparison withno magnetic annular body 8 mounted. Accordingly, the magnetizing forcecan be adjusted by attaching or detaching the magnetic annular bodies 8to or from the first yoke 1, resulting in changing the magnetic fluxdensity of the magnetic pipe.

FIG. 6 is a diagram showing the configuration of an eddy current flawinspection apparatus 200 according to an embodiment of the presentinvention. The eddy current flaw inspection apparatus 200 according tothe present embodiment is used for inspecting flaws in a magnetic pipe300 composed of a magnetic material. The eddy current flaw inspectionapparatus 200 includes the eddy current flaw detection probe 100according to the present embodiment as well as an AC power supply 201,signal output means 202, and flaw judging means 203.

In the eddy current flaw inspection apparatus 200, the eddy current flawdetection probe 100 moves along the inner peripheral surface of themagnetic pipe 300.

The AC power supply 201 applies an alternating voltage onto the sensingcoil 6 of the eddy current flaw detection probe 100. The signal outputmeans 202 outputs a voltage change induced on the sensing coil 6 whenthe alternating voltage is applied by the AC power supply 201, as a flawdetection signal which is an electric signal composed of anX-directional component signal and a Y-directional component signal. Inthe present embodiment, an eddy current flaw detector (AV100SE,manufactured by GE Inspection Technologies) is employed as the signaloutput means 202.

The flaw judging means 203 is a personal computer or the like andreceives the flaw detection signal outputted from the signal outputmeans 202 so as to determine the presence of a flaw in the magnetic pipe300 when the maximum of the X-directional component signal or themaximum of the Y-directional component signal in the flaw detectionsignal exceeds a threshold set up for each component signal in advance.

By adopting the configuration that the determination result obtainedfrom the flaw judging means 203 is displayed on display means such as adisplay panel, the operator who operates the eddy current flawinspection apparatus 200 can recognize the occurrence of a flaw in themagnetic pipe 300. Alternatively, by adopting the configuration that thewaveforms of the flaw detection signal outputted from the signal outputmeans 202 are displayed on the display means, the operator may judge byvisual determination on the displayed waveforms whether the maximum ofthe X-directional component signal or the maximum of the Y-directionalcomponent signal exceeds the threshold.

The eddy current flaw inspection apparatus 200 according to the presentembodiment employs the eddy current flaw detection probe 100 accordingto the present embodiment. Thus, even for magnetic pipes 300 of mutuallydifferent specifications in the magnetic permeability or the thickness,flaw inspection is achieved by using the eddy current flaw detectionprobe 100 whose specification is optimized for the specification of themagnetic pipe 300. Thus, the eddy current flaw inspection apparatus 200according to the present invention achieves accurate flaw inspection ofthe magnetic pipe 300 as an inspection target.

The invention claimed is:
 1. An eddy current flaw detection probecomprising: a first yoke having a hollow-cylindrical shape and composedof a magnetic material; a second yoke having a hollow-cylindrical orpillar shape, composed of a magnetic material, and inserted in a freelyinsertable and extractable manner along an inner peripheral surface ofthe first yoke; a first permanent magnet mounted along an outerperipheral surface of one end part of the axis direction of the firstyoke and polarized in a radial direction of the first yoke; a secondpermanent magnet mounted along an outer peripheral surface of the otherend part of the axis direction of the first yoke, polarized in a radialdirection of the first yoke, and magnetized such that the magnetic polesare different from those of the first permanent magnet; and a sensingcoil wound along the outer peripheral surface in a center part of theaxis direction of the first yoke and detecting a change in a magneticflux generated by the first permanent magnet and the second permanentmagnet.
 2. The eddy current flaw detection probe according to claim 1,further comprising a magnetic annular body having an annular shape,composed of a magnetic material, and mounted in a freely attachable anddetachable manner along the outer peripheral surface of the first yoke.3. An eddy current flaw inspection apparatus for inspecting a flaw in apipe composed of a magnetic material, comprising: the eddy current flawdetection probe according to claim 1 moved along the inner peripheralsurface of the pipe; an AC power supply for applying an alternatingvoltage onto the sensing coil of the eddy current flaw detection probe;signal output means for outputting as an electric signal a voltagechange induced on the sensing coil when the alternating voltage isapplied by the AC power supply; and flaw judging means for judging thepresence or absence of a flaw in the pipe on the basis of the electricsignal outputted from the signal output means.